There has been considerable interest in the design and construction of a superconducting transmission line system over the last quarter century. Because the resistance of a superconductor cooled below its superconducting transition temperature is zero for direct current, the power can be transmitted without I.sup.2 R loss, making such a system very attractive because of its high efficiency. Conceptual designs and studies have been performed, and actual prototype systems have been built and tested. Such prior system have typically used superconducting material with a transition temperature far below the boiling point of liquid nitrogen (77 K.), thus requiring the use of liquid helium which is very expensive.
Major projects using short lengths of superconducting transmission line have been built and tested at Brookhaven, Upton, N.Y.; LASL, Los Alamos, N.Mex.; and ATF, Graz, Austria. Each project used a helium liquefier providing helium which was pumped into the line. For further information concerning the structure and operation of each project, reference may be made to "Cryo-Electric Testing of a 1000 MVA Superconducting Power Transmission System", Forsyth, Advances In Cryogenic Engineering, Proceedings of CEC, Colorado Springs, Vol. 29, 1983, pp. 113-121; "Current Test of a DC Superconducting Power Transmission Line", Edeskuty et al., IEEE Transactions on Magnetics, Vol. MAG-17, No. 1, June 1981, pp. 161-164; and "Practical Conclusions From Field Trials Of A Superconducting Cable", Klaudy et al., IEEE Transactions on Magnetics, Vol. MAG-19, No. 3, May 1983, pp. 656-661.
Cryoresistive cables have also been proposed for electric power distribution. The cores of such cables, while not superconducting, have decreased resistance with decreased operating temperature. An example of such cable includes a liquid nitrogen cooled aluminum conductor with its design patterned after a conventional oil-cooled hollow aluminum cable in which oil is pumped through the cable. For further information concerning the structure and operation of such cryoresistive cables, reference may be made to "Cryogenic Power Transmission", Minnich et al., Cryogenics, June 1969, pp. 165-176, and "The Results Of Work Carried Out In The USSR On Creation Of Superconducting And Cryoresistive Cables for Electric Power Lines", Meshchanov et al., IEEE Transactions On Magnetics, Vol. MAG-19, No. 3, May 1983, pp. 662-667.